1. Field of the Invention
The present invention relates to a semiconductor memory device, in particular, in relates to a semiconductor memory device providing redundant cells.
2. Description of the Related Art
Recently, the capacity of semiconductor memory devices has been and is being greatly increased, and this increase in the capacity of the semiconductor memory device leads to a high probability that some of the memory cells therein will be defective; thus rendering, the entire memory defective. To alleviate this problem, there is known a method for providing redundant memory cells and of using these redundant memory cells to replace defective memory cells.
The provision of a large number of redundant cell groups, is useful if a large number of defective memory cells are produced in a cell block. However, where defective memory cells are not produced, the provision of so many redundant cell groups is unnecessary, in that there is no use for them. Thus, usually, only one column or 2 to 3 columns of the redundant cell groups are provided. Further, in the conventional system, the redundant cells groups are linked exclusively to individual cell blocks.
When a redundant cell group is linked to an individual cell block, the following problems arise. That is, assuming that one column of redundant cells is provided for each cell block, and that two defective memory cells linked to a different column are produced in the first cell block but no defective cells are produced in the second cell block. In this case, althrough there are two redundant cell groups and two defective memory cells, the two redundant cell groups are linked to different cell blocks, and therefore, the redundant cell group in the first cell block cannot be used for the second cell block. Thus, only one redundant cell group can be used for replacing the defective cells, and therefore, another defective memory cells cannot be replaced and the functioning of the memory device is deteriorated.